Traditional MRI is an important 3D medical imaging modality used for soft-tissue imaging in medical diagnosis. The primary problem with MRI is the slow scanning time, and in MRI with high magnetic fields, the presence of high magnetic fields introduces many problems. Scanning of a 3D object is done using slice-selection along the Z-axis based on Radio Frequency (RF) pulse with a particular frequency, and frequency encoding of voxel positions along the x-axis, and phase-encoding along the y-axis. In the case of Ultra-Low Field (ULF) MRI where the field strengths are in micro-Tesla range comparable to the earth's magnetic field, RF pulses need not be used. Position along the x-axis is encoded by frequency and phase encoding is used along both the y-axis and the z-axis. In parallel MRI (pMRI) schemes such as SENSE, SMASH, and GRAPPA, partial parallelism is achieved with RF sensor coils having different spatial sensitivities.
MDI can be combined with MRI, particularly ultra-low field (ULF) MRI to provide both T1 and T2 weighted images with a scanning time that is much less than MRI in prior art. ULF magnetic fields are measured using highly sensitive devices such as Superconducting Quantum Interference Devices (SQUIDS) or atomic magnetometers. Using such measured data, one can reconstruct a 3D image of the magnetic dipole patterns in an object such as soft tissue in a human body. More recently, ULF MRI has been demonstrated to be effective for security screening of passengers at airports, particularly for detecting explosive liquids hidden inside clothes of passengers.
This invention discloses methods and apparatuses for 3D imaging of objects based on Field Paradigm (FP) proposed recently by the author of this invention. Field Paradigm is a novel unified framework for several 3D medical imaging modalities including MRI and MDI. It is based on the Field Image Principle (FIP) stated as “the field intensity distribution in a 3D volume space uniquely determines the 3D density distribution of the field emission source and vice versa”. This principle has not been realized, recognized, or exploited in the past in 3D medical imaging. Theoretical soundness of this principle is easily verified. Experimental verification has been done on rudimentary cases through computer simulation.
Field Image Tomography (FIT) is a theoretical framework based on Field Paradigm that provides computational algorithms for reconstructing 3D density distribution of field emission sources from the measured 3D field image data. In MDI magnetic dipoles in voxels act as sources of magnetic field in a 3D volume space. In MRI the magnetic dipoles similar to those in MDI are present. Each dipole is the vector sum of the magnetic moments of a large number of nuclei processing at Larmor frequency. In the beginning, these nuclei have random phases and therefore the total magnetic moment will be along the polarizing magnetic field. In MRI, these nuclei are made to spin in-phase using an RF pulse or a readout or measurement magnetic field Bm applied perpendicular to the polarizing magnetic field (after the polarizing field is turned off). Therefore the total magnetic moment vector will precess at the Larmor frequency. This results in a transverse time-varying magnetic flux that induces electric current in a measuring induction coil or a SQUID sensor. It is also possible to measure and use flux intensity instead of rate of change of flux with time for imaging.
Field Image Tomography, unlike techniques in prior art, is information-efficient in the sense that it exploits all available information that is useful for 3D image reconstruction. It is based on measuring field flux in a 3D volume space along multiple different directions instead of measurements on a 2D surface along a single direction at nearly fixed radial distance from the field emission source. In particular, field intensity produced by a small emission source decays with radial distance, and this characteristic of field propagation in a 3D volume space is exploited to determine the 3D density distribution of an emission source. Also, at each point, field intensity may be measured along multiple directions at each point for optimal image reconstruction results. A trade-off may be possible between the extent or length of the radial direction where measurements are made and the number of different directions along which measurements are made.
The data measured in the present invention captures all the available information and facilitates a computationally efficient solution to the 3D image reconstruction problem. This is unlike prior art where measurements are made only on a surface at a nearly constant radial distance from the center of the target object, and along a single direction. Therefore useful, and available data is ignored and not measured in prior art leading to suboptimal and inefficient (in terms of information usage) methods and apparatuses.
The magnetic dipole density distribution in both MDI and ULF MRI produce a magnetic field that is measured by high-sensitivity sensors such as SQUIDS (Superconducting Quantum Interference Devices) or atomic magnetometers. The measured values of the magnetic field intensity or its spatial derivatives are used to compute the 3D image of the dipole density distribution. This 3D image is useful in the diagnosis of many ailments. Magnetic fields are preferable in medical imaging in comparison with X-rays because magnetic fields are harmless and pass through body tissue and bones without much attenuation and distortion. The present invention provides methods and apparatuses for 3D imaging of soft tissue that are significantly faster than current MRI techniques.
The methods and apparatuses in prior art rely solely on data measured roughly on a surface and therefore they do not fully exploit all the available information. They ignore additional information available in the variation of magnetic field intensity with radial distance and along different directions in a 3D volume space around a target object.
This invention is based on measuring the magnetic field in an extended 3D volume space, not just on a surface at a nearly constant radial distance from the target object as in prior art. Magnetic field intensity measurements (or its derivatives) are made in a 3D volume space that extends substantially along the radial direction from the center of the target object. Further, magnetic field intensity measurement at each point may be made along multiple directions. This has the effect of measuring a 3D vector field in a 3D space at different distances and along different directions at each point. This type of measurement effectively captures almost all available information that is useful in 3D image reconstruction.
Field Paradigm has been applied recently by the author of this invention to 3D medical imaging in four other cases: Single-Photon Emission Computed Tomography (SPECT), high-field (0.01 to 10 Tesla) Magnetic Resonance Imaging (MRI), Magnetoencephalography (MEG) and Magnetocardiography (MCG). A detailed description of these applications are provided in the following U.S. patent applications filed by the author of this invention recently:    1. M. Subbarao, “Method and apparatus for high-sensitivity Single-Photon Emission Computed Tomography”, U.S. patent application Ser. No. 12/586,863, Date Sep. 29, 2009.    2 M. Subbarao, “Field Image Tomography for Magnetic Resonance Imaging”, U.S. patent application Ser. No. 12/658,001, Date Feb. 1, 2010.    3. Methods and apparatuses for 3D Imaging in Magnetoencephalography and Magnetocardiography, U.S. patent application Ser. No. 12/924,959, Date Oct. 9, 2010.
The contents of the three patent applications listed above and the following published papers on Ultra Low-field MRI and Magnetic Polarization Imaging (MPI) in their entirety are incorporated herein by reference:    1. V. S. Zotev, A. N. Matlashov, P. L. Volegov, I. M. Savukov, M. A. Espy, J. C. Mosher, J. J. Gomez, R. H. Kraus Jr., “Microtesla MRI of the human brain combined with MEG”, Journal of Magnetic Resonance, 194 (2008) 115-120.    2. J. O. Nieminen, M. Burghoff, L. Trahms, R. J. Ilmoniemi, “Polarization encoding as a novel approach to MRI”, Journal of Magnetic Resonance 202 (2010) 211-216.
The prior art described in the above two papers and the references therein do not use a method that involves measuring magnetic field data in a 3D volume space that extends substantially along the radial direction, nor do they describe an apparatus that has a means for measuring the magnetic field around an object in a 3D volume space that extends substantially along the radial direction. In addition, the methods and apparatuses disclosed in these and other related papers always use frequency encoding, phase encoding, or polarization encoding. MDI does not require any such encoding but it can be combined with those encoding schemes for various trade-offs such as speed of imaging and number of sensor elements to measure magnetic fields. Therefore, as explained earlier, the methods and apparatuses in the above 2 papers and related papers suffer from many disadvantages such as being slow and less accurate as they do not measure all available information that is useful in 3D imaging.